1. Field of the Invention
The present invention relates to an electric power steering apparatus for producing a steering assist force by an electric motor.
2. Description of Related Arts
Electric power steering apparatuses (EPS) for motor vehicles comprise electric motors for steering assist and reduction gear mechanism for amplifying outputs of the electric motors. For example, the electric power steering apparatus arranged in a steering column comprises a worm shaft and a worm wheel meshed with the worm shaft as the reduction gear mechanism. The worm shaft and the worm wheel decelerate the rotation of an output shaft of the electric motor so that an output of the electric motor is amplified. The amplified output of the electric motor is transmitted to a steering mechanism, to assist a steering operation.
An appropriate backlash is provided in mesh between the worm shaft and the worm wheel. However, the backlash may cause the worm shaft to vibrate when a vehicle travels on a bad road, for example, so that a tooth striking sound (rattle sound) may be produced.
In order to prevent the tooth striking sound from being produced, the amount of the backlash must be strictly adjusted within a range of processing accuracy of each of components. When the worm shaft and the worm wheel are assembled, each of the components has been conventionally selected depending on the degree of variation in dimensional accuracy to combine the components whose system accuracies are proper to each other (so-called matching assembling). However, it takes time and labor to perform this working, so that the manufacturing cost is high.
Even if the backlash is set in an appropriate range in the early stages by carrying out the matching assembling, the backlash may be increased due to wear of a tooth portion due to changes with time to produce an abnormal sound.
In the invention disclosed in Japanese Unexamined Patent Publication No. 2005-254864, a curved plate spring is interposed between a bearing for supporting one end of a worm shaft and the inner periphery of a bearing hole formed in a housing. The curved plate spring causes the worm shaft to be elastically urged toward the worm wheel, thereby making a backlash zero.
Specifically, the curved plate spring has an annular main body having ends and covering the outer periphery of an outer ring of the bearing and a pair of hook-shaped elastic tongues supported in a cantilevered shape by each of the ends of the main body. The tip of each of the elastic tongues is elastically deflected by being abutted against the bottom of a recess of the bearing hole. The worm shaft is urged toward the worm wheel through the bearing by a repulsive force of the deflection.
A base end of each of the elastic tongues is abutted against an edge of an inlet to the recess of the bearing hole, thereby regulating the rotation of the curved plate spring.
In the invention disclosed in the above-mentioned gazette, however, a stress caused by the deflection of the elastic tongue is the highest at the base end of the elastic tongue. Further, a load stress for regulating rotation is added to the base end. Consequently, an excess stress is applied to a part of the curved plate spring, particularly to the base end. As a result, the durability of the curved plate spring may be degraded.
An object of the present invention is to provide an electric power steering apparatus capable of preventing noises from being produced over a long time period.